In recent years, developments of electroluminescent elements using organic thin films (i.e. organic electroluminescent elements) have been carried out.
Among them, in order to improve the performance of an organic electroluminescent element, development of an organic electroluminescent element having a structure wherein organic thin films are laminated in a multilayer construction (hereinafter referred to as a “multilayer laminate structure”) has been actively carried out.
In an organic electroluminescent element having a multilayer laminate structure, at least two layers different in composition are present. At their interface, there is a difference in the ionization potential or electron affinity, which becomes a hindrance at the time of interlayer injection of an electric charge (holes or electrons). This has been a cause of an increase of a driving voltage in an organic electroluminescent element.
Meanwhile, as a method for forming an organic thin film in an organic electroluminescent element, a vacuum vapor deposition method and a wet film-forming method may be mentioned.
In development of an organic electroluminescent element employing a vacuum vapor deposition method, e.g., in Patent Document 1, it is disclosed that by using a co-evaporation method as the vacuum vapor deposition method, a multilayer laminate structure is formed to reduce the above-mentioned hindrance in injection of an electric charge.
On the other hand, the wet film-forming method has merits such that no vacuuming process is required, film formation in a large area is easy, and it is easy to incorporate a plurality of materials having various functions as mixed to a single layer (composition). For example, Patent Document 2 or 3 discloses an invention of an organic electroluminescent element wherein a hole injection layer and a light-emitting layer are formed by a wet film-forming method, but it had problems such that as compared with an element formed by a vacuum deposition method, the current efficiency or power efficiency is low, the driving voltage is high, and the luminescent efficiency is low.